Try the simple stuff first
I know this sounds a bit condescending, but have you checked the temp. gauge? The sender and/or gauge may be off.
You may want to verify the actual guage before going on an expidition on the engine. I kid you not, I've seen and heard folks that were ready to swap out engines due to: no fuel or plugged filters (never starts/runs over 1,000 rpms), sea cock innards rusted off (over heats in 2 minutes), plugged strainer (over heats in 15 minutes), etc.
Start simple and work backwards before thinking about a "rebuild".
Some things to consider/check:
A-1. Verify temp gauge and sender (!)
1. Clean bottom
2. Correct prop (with nothing wrapped around it).
3. Cooling system is clean (when's the last time you ran some muriatic acid through the raw water side and the last time you purged and changed the coolant?).
4. Clean AND correct fuel/air/oil filters (and clean fuel for that matter which isn't 2 years old).
5. Check the strainer for gunk (plastic bags are notorious problems)
6. Rebuilt/exchange the water pumps - the raw water pump has wear components that require replacement now and then (cam, impeller, cover plate, gasket, seals, etc.).
7. Open up the heat exchager and had a look at what's inside (rubber impellor parts and bits of plastic will plug it up).
8. Inspect the raw water/coolant/exhaust hoses for any collapsed or bent sections. Also. check for bulges in rubber hoses where thay clamp on to engine (especially engine coolant ones) as that may indicate a blocked or severely restricted coolant passage.
9. The shaft is not binding (cutlass bearing, alignment, shaft packing, etc.).
10. Exhaust system is in good health (aqua lift, hoses, ect.)
11. Thermostat is operating correctly.
12. no vacuum leaks in the cooling system hoses (especially raw water side - hose clamps are tight, etc.).
13. Basic tune up - adjust valves, injector cleaning, timing of injector pump, etc.
14. clutch is operating correctly and not binding - tansmission is operating correctly (correct oil, topped off/not to full, etc.)
15. Oil changed and of correct viscosity and API code.
16. Disconnect engine from sea cock and strainer and operate it from a bucket of water and see if that helps
17. Operating in very warm water (Bahamas, shallow florida waters, etc.).
18. Oh yeah..... ---->>fluids are at the correct level!
19. Air in the cooling system
That should give you some areas to persue before you start pulling your hair out and then the engine. Usually an engine will fail catastrophicly or give you clear signs of requyiring a rebuild such as consuming lots of oil, strange metallic/clunking noises, smoking severely (exhaust and breather), little to no power, etc.
Good luck and there are quite a few items you can check on the list yourself before calling a mechanic. As can be expected, I've seen folks paying someone else to tell them their strainer is plugged, they are out of fuel, a water hose is kinked or the coolant system needs topping off/changing....that's good money better spent elsewhere!
Cheers,
John M.